Here's some answers on this. The call blockers & phones now have area code blocking which is good since as you mentioned, they only change by a digit or two. This I have found works the best. And they will change area codes but not so very quickly as they do the number they are spoofing.
Robocaller "Carmen" -- Credit Card Services AGAIN! Been receiving 3-4 calls a week from this robocaller, ID displays a variety of phone numbers,towns & states, i.e. Blockbuster Ent. I have been on the Do Not Call List for 5+ years.
I get these calls at least two times a day saying they can low my intrest rate on my Credit Cards, Well I currently Do not own a Credit Card at all so i went along with it and said ooh yea so they asked how much debt i am in and I said non and i said by the way I do not even own a credit card. The min i said that they Hung up, then two hrs later they call again, this is getting ridiculous seeing i am on the do not call list! this has to be a scam!!! Beware!!!
I got a call from this number, but it came up on my caller ID as someone actually in my phone book. When I looked closely the phone numbers are off by 1 number (the number in my book is 319-225-1839) but the name as it appears in my phonebook (think "Mike the PBJ Guy") is what showed up. This is new levels of low. I answered thinking it was Mike, only to be greeted by a robo call to lower my interest rates on my credit cards.